Accessing inactive union member and undefined behavior?

Question

I was under the impression that accessing a union member other than the last one set is UB, but I can\'t seem to find a solid reference (other than answers clai

Answer 1

I well explain this with a example.
assume we have the following union:

union A{
   int x;
   short y[2];
};

I well assume that sizeof(int) gives 4, and that sizeof(short) gives 2.
when you write union A a = {10} that well create a new var of type A in put in it the value 10.

your memory should look like that: (remember that all of the union members get the same location)

       |                   x                   |
       |        y[0]       |       y[1]        |
       -----------------------------------------
   a-> |0000 0000|0000 0000|0000 0000|0000 1010|
       -----------------------------------------

as you could see, the value of a.x is 10, the value of a.y1 is 10, and the value of a.y[0] is 0.

now, what well happen if I do this?

a.y[0] = 37;

our memory will look like this:

       |                   x                   |
       |        y[0]       |       y[1]        |
       -----------------------------------------
   a-> |0000 0000|0010 0101|0000 0000|0000 1010|
       -----------------------------------------

this will turn the value of a.x to 2424842 (in decimal).

now, if your union has a float, or double, your memory map well be more of a mess, because of the way you store exact numbers. more info you could get in here.